The membrane of a eukaryotic cell has, as integral components, glycoproteins which are receptors for a variety of regulatory molecules. Immunoglobulins are the specific membrane receptors for the antigens which stimulate proliferation and differentiation in B lymphocytes. Chemical characterization of these receptor immunoglobulins is of enormous interest, but the small amount of material and its relative insolubility in aqueous solutions have made this a difficult task. The recent development of microsequencing techniques based on detergent solubilization of the membrane molecules has made the detailed structural analysis of these molecules feasible. We intend to employ these techniques in the partial determination of the primary structure of membrane immunoglobulins from mouse B cell tumors. This information will be compared with similar information determined for secreted immunoglobulin and for membrane immunoglobulins from normal B cells. This detailed structural analysis will be a foundation from which to examine the functional interactions of the membrane immunoglobulins with other membrane molecules, using reversible cross-linking reagents. Changes in these interactions will be documented by comparing normal B cells, B cells stimulated to divide by mitogens, subpopulations of B cells at different developmental stages and B cell tumors, this system presents a unique opportunity for the study of chemical structure and function of an accessible, membrane-bound receptor which is intimately involved in the control of proliferation and differentiation in normal cells.